In this study, we aim to identify a relationship, if any, between the numbers of snails found, whether alive or dead, banded or un-banded in relation to where they are situated in the field. The results that we obtain from our research will enable us to conclude whether genetic drift or natural selection have an effect upon the distribution of snails. Cepaea nemoralis, is a species of snails that occupy a range of habitats, such as woodland, grassland, hedgerows and garden shrubs in the UK. The highly variable colours consisting of yellows, pinks and browns, as well as up to five horizontal banding patterns on their shells make great observable phenotypic traits. This polymorphism is related to biodiversity, genetic variation and adaptation, and results from evolutionary processes. The colours and banding patters is heritable and its frequency within a population can be modified by natural selection. The genetic make-up determines the morph of Cepaea nemoralis, and it is for this reason these species are used to understand evolutionary biology.
When arriving at the nature reserve, we immediately noticed that the size and thickness of vegetation varied. Some regions, i.e. towards the footpath from which we entered the reserve, had very fierce vegetation, compared to the less thick shrubs found randomly scattered along the open space of the field. We decided this would be an excellent variable, as there may be a distinct pattern in the type and number of snails found within the two distinctive areas.
Starting from the left side of the footpath, we worked our way across – gathering snail counts from a total of 8 different sites; 4 consisting of thin vegetation (small, less vigorous shrubbery) and 4 consisting of thick vegetation (larger, more compact shrubbery). We specifically collected data on the snails whilst ensuring that we recorded our data on high grounds. This would certify that the relief of the landscape would not affect our results. Locating our snail species, we carefully recorded the colour, band number, and whether they were alive or dead adults, these distinctive traits would allow a comparison between the distributions of snails and allows us to indentify the specific sort found there. This, along with many numbers of sites, would help us distinguish whether genetic drift or selection has played a role within the distribution of snails.
In the past similar but greater investigative studies have been carried out. For example in 2009, snail shells were collected from a thrush’s anvil as scientists were attempting to discover a trend in the number of differently patterned and coloured snails hunted and eaten by the thrush at different months throughout the year. Thrushes are passerine birds which smash snails on particular stones called anvils before eating them.
Month
Collected shells that were yellow (%)
April
42
June
22
Banded Snail Popluations Graph 400
From the data analysis, they found that in April of 2009 the percentage of yellow snail shells collected was 42%; however in June of the same year the percentage of yellow snail shells collected was significantly lower as it was 22%. During spring, the un-banded, plain yellow snail shells seem to be better camouflaged against the new leaves and grass. Large numbers of snail shells were discovered in the woodland with one or more band, as well as great numbers being found in grassland and hedgerows that had several bands on their shell. Overall, there were a higher percentage of yellow snails in hedgerows and grassland. If the amount of woodland were to decrease, then the numbers of yellow snails would increase as a result.
There are a minimum of three selective pressures that could potentially affect the population sizes of the snails. Changes in the population characteristics may occur as a result of human activities such as burning down the woodland or cutting the tall trees (clearing the woodland). This is because colour and number of bands influence their ability to camouflage in various habitats.
Thrush populations play a key role in determining the snail population size as they are one of the major predators. They ideally hunt and feed on those which can be seen more easily, however as their population size have reduced over the recent years, the benefits of some camouflaged patterns may be reduced. Darker snail shells also have a selective advantage; they warm up in daylight much faster, resulting in greater activity and hence have an advantage in finding food. Nevertheless, with global warming increasing, this will be less of an advantageous trait.
DISCUSSION
We predict that there will be a higher percentage of yellow snails with fewer bands in the grassland (thin vegetation) when compared to the thick vegetation where pink/brown snails with a greater number of bands are expected to be in greater abundance. Hence our null hypothesis being that there is no significant difference between the distribution of yellow 0-3 bands, yellow 4-5 bands, pink+brown 0-3 bands, and pink+brown 4-5 bands snails in the high ground thin vegetation and high ground thick vegetation.
We believe this because it seems that fewer-banded, yellow shelled snails should be better camouflaged within the leaves and grass, unlike the pink and brown, either banded or unbanded, snails which would clearly stand out in the open fields making them easier to be preyed upon. Therefore, there should be a significant difference between these two groups of data with a greater frequency of yellow snails with 0-3 bands within thin vegetation populations.
By carrying out a chi squared calculation to analyse our results we would attain knowledge of any significant difference between our variables. By conducting a chi squared test we have achieved a value of 6.78. Our statistical test is based from a chi squared distribution at two degrees of freedom; this is because we have three colour variables against two habitat variables. Referring to the chi-squared distribution for 2 degrees of freedom we can see that the probability of observing this difference (or a much greater difference) is approximately 4.61 if the frequencies of the coloured snails in the four populations are greatly different at the two sites.
The calculated value for the chi squared test for the set of data is 6.77 which is greater than the stated criteria for the statistical significance of 4.61, therefore we can provisionally reject the null hypothesis at the p=0.1 level and believe there is significant difference between the distribution of yellow 0-3 bands, yellow 4-5 bands, pink+brown 0-3 bands, and pink+brown 4-5 bands snails in the high ground thin and thick vegetations which cannot be due to chance alone.
To make our experimental findings stand firm, we decided to analyse our results at the level of p=0.05, which has the critical value of 5.99 at 2 degrees of freedom; this is still smaller than our calculated value. The lower critical value of the chi squared distribution is 0.103; so an x2 value greater than 0.103 would occur in less than 5% of experiments conducted if the null hypothesis were true. We can therefore conclude, with a high confidence level, that the difference in the phenotypic frequencies examined cannot be due to sampling error.
Though we cannot rule out the probability of sampling error completely, we can predict other possible reasons for which this diversity is seen; it could be due to either genetic drift, natural selection, or both. There is a change in allele frequency within the thick and thin vegetation populations suggesting the different habitats impose different selection pressures. It may also be due to the lack of gene flow between the populations which is causing the dissimilar phenotypic frequencies within the different habitats.
Cepaea nemoralis, is a species of snails that occupy a range of habitats, such as woodland, grassland, hedgerows and garden shrubs in the UK. The highly variable colours consisting of yellows, pinks and browns, as well as up to five horizontal banding patterns on their shells make great observable phenotypic traits. This polymorphism is related to biodiversity, genetic variation and adaptation, and results from evolutionary processes. The colours and banding patters is heritable and its frequency within a population can be modified by natural selection. The genetic make-up determines the morph of Cepaea nemoralis, and it is for this reason these species are used to understand evolutionary biology.
When arriving at the nature reserve, we immediately noticed that the size and thickness of vegetation varied. Some regions, i.e. towards the footpath from which we entered the reserve, had very fierce vegetation, compared to the less thick shrubs found randomly scattered along the open space of the field. We decided this would be an excellent variable, as there may be a distinct pattern in the type and number of snails found within the two distinctive areas.
Starting from the left side of the footpath, we worked our way across – gathering snail counts from a total of 8 different sites; 4 consisting of thin vegetation (small, less vigorous shrubbery) and 4 consisting of thick vegetation (larger, more compact shrubbery). We specifically collected data on the snails whilst ensuring that we recorded our data on high grounds. This would certify that the relief of the landscape would not affect our results. Locating our snail species, we carefully recorded the colour, band number, and whether they were alive or dead adults, these distinctive traits would allow a comparison between the distributions of snails and allows us to indentify the specific sort found there. This, along with many numbers of sites, would help us distinguish whether genetic drift or selection has played a role within the distribution of snails.
In the past similar but greater investigative studies have been carried out. For example in 2009, snail shells were collected from a thrush’s anvil as scientists were attempting to discover a trend in the number of differently patterned and coloured snails hunted and eaten by the thrush at different months throughout the year. Thrushes are passerine birds which smash snails on particular stones called anvils before eating them.
From the data analysis, they found that in April of 2009 the percentage of yellow snail shells collected was 42%; however in June of the same year the percentage of yellow snail shells collected was significantly lower as it was 22%. During spring, the un-banded, plain yellow snail shells seem to be better camouflaged against the new leaves and grass. Large numbers of snail shells were discovered in the woodland with one or more band, as well as great numbers being found in grassland and hedgerows that had several bands on their shell. Overall, there were a higher percentage of yellow snails in hedgerows and grassland. If the amount of woodland were to decrease, then the numbers of yellow snails would increase as a result.
There are a minimum of three selective pressures that could potentially affect the population sizes of the snails. Changes in the population characteristics may occur as a result of human activities such as burning down the woodland or cutting the tall trees (clearing the woodland). This is because colour and number of bands influence their ability to camouflage in various habitats.
Thrush populations play a key role in determining the snail population size as they are one of the major predators. They ideally hunt and feed on those which can be seen more easily, however as their population size have reduced over the recent years, the benefits of some camouflaged patterns may be reduced. Darker snail shells also have a selective advantage; they warm up in daylight much faster, resulting in greater activity and hence have an advantage in finding food. Nevertheless, with global warming increasing, this will be less of an advantageous trait.
DISCUSSION
We predict that there will be a higher percentage of yellow snails with fewer bands in the grassland (thin vegetation) when compared to the thick vegetation where pink/brown snails with a greater number of bands are expected to be in greater abundance. Hence our null hypothesis being that there is no significant difference between the distribution of yellow 0-3 bands, yellow 4-5 bands, pink+brown 0-3 bands, and pink+brown 4-5 bands snails in the high ground thin vegetation and high ground thick vegetation.
We believe this because it seems that fewer-banded, yellow shelled snails should be better camouflaged within the leaves and grass, unlike the pink and brown, either banded or unbanded, snails which would clearly stand out in the open fields making them easier to be preyed upon. Therefore, there should be a significant difference between these two groups of data with a greater frequency of yellow snails with 0-3 bands within thin vegetation populations.
By carrying out a chi squared calculation to analyse our results we would attain knowledge of any significant difference between our variables. By conducting a chi squared test we have achieved a value of 6.78. Our statistical test is based from a chi squared distribution at two degrees of freedom; this is because we have three colour variables against two habitat variables. Referring to the chi-squared distribution for 2 degrees of freedom we can see that the probability of observing this difference (or a much greater difference) is approximately 4.61 if the frequencies of the coloured snails in the four populations are greatly different at the two sites.
The calculated value for the chi squared test for the set of data is 6.77 which is greater than the stated criteria for the statistical significance of 4.61, therefore we can provisionally reject the null hypothesis at the p=0.1 level and believe there is significant difference between the distribution of yellow 0-3 bands, yellow 4-5 bands, pink+brown 0-3 bands, and pink+brown 4-5 bands snails in the high ground thin and thick vegetations which cannot be due to chance alone.
To make our experimental findings stand firm, we decided to analyse our results at the level of p=0.05, which has the critical value of 5.99 at 2 degrees of freedom; this is still smaller than our calculated value. The lower critical value of the chi squared distribution is 0.103; so an x2 value greater than 0.103 would occur in less than 5% of experiments conducted if the null hypothesis were true. We can therefore conclude, with a high confidence level, that the difference in the phenotypic frequencies examined cannot be due to sampling error.
Though we cannot rule out the probability of sampling error completely, we can predict other possible reasons for which this diversity is seen; it could be due to either genetic drift, natural selection, or both. There is a change in allele frequency within the thick and thin vegetation populations suggesting the different habitats impose different selection pressures. It may also be due to the lack of gene flow between the populations which is causing the dissimilar phenotypic frequencies within the different habitats.
Syeda Arifa Sultana
Dipesh Patel
Salman Khan
Alveera Hasan
Tayyabah Yousaf